The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Maximum power deliverable by a cascode common-source (CS) amplifier is limited by the maximum stress that the device can tolerate. One stress parameter relates to a drain-source voltage VDS across a transistor. Referring now to FIG. 1, a cascode CS amplifier may be used to increase the maximum power deliverable in a single stage. The cascode CS amplifier includes a transistor N1 and a transistor N2. A control terminal of the transistor N2 may be connected to a bias signal Vb. A first terminal of the transistor N1 is connected to a second terminal of the transistor N2. A second terminal of the transistor N1 is connected to a reference potential such as ground. A control terminal of the transistor N1 receives an input voltage Vin and an output current Iout is generated.
The cascode CS amplifier transforms an input voltage into an output current. The voltage at the output of the cascode CS amplifier depends on a load. When the input swings low, the output will swing high due to the inverting nature of the cascode CS amplifier. In this state, the transistors N1 and N2 will turn off. In order for the transistor N2 to turn off, the source voltage only needs to rise to the level of VG2−VTH2, where VG2 is a gate bias voltage of the transistor N2 and VTH2 is the threshold voltage of the transistor N2.
For example, if a drain voltage VD of the transistor N2 has a quiescent value of 3.6 V, the quiescent drain voltage VD of the transistor N1 is 1.8 V, and the quiescent gate voltage VG of the transistor N1 is 0.6 V, then the quiescent gate voltage VG of the transistor N2 should be approximately 2.4 V. If the threshold voltage VTH is 0.4 V, then the maximum voltage that the drain voltage VD of the transistor N1 can swing to is approximately 2.4−0.4=2.0 V. If the output voltage of the cascode CS amplifier swings to 7.2 V (which may occur in an inductively loaded cascode CS amplifier), then the drain-source voltage VDS across the transistor N1 will reach a maximum of 2.0 V, while the drain-source voltage VDS of the transistor N2 will reach a maximum of 5.2 V. This large voltage across the transistor N2 can cause long term stress, and limit the useful lifetime of the device.